Add a passthrough directory feature. (#76)

This is only compiled in when a `dev` build tag is specified (see #75)
for safety. Attempting to use the flag without that tag at build time
will fail. However, it is always available within the codebase itself so
that tests can be written against this functionality in a programmatic
way.

Manual testing was performed; no plans to add automated ones as this
isn't production-necessary nor exported functionality.

internal/servers/controller/handler.go

@@ -5,6 +5,7 @@ import (
 	"encoding/json"
 	"net/http"
 	"path"
+	"path/filepath"
 	"strings"
 	"time"
 
@@ -30,6 +31,20 @@ func (c *Controller) handler(props HandlerProperties) http.Handler {
 	// Create the muxer to handle the actual endpoints
 	mux := http.NewServeMux()
 
+	if c.conf.RawConfig.PassthroughDirectory != "" {
+		// Panic may not be ideal but this is never a production call and it'll
+		// panic on startup. We could also just change the function to return
+		// an error.
+		abs, err := filepath.Abs(c.conf.RawConfig.PassthroughDirectory)
+		if err != nil {
+			panic(err)
+		}
+		c.logger.Warn("serving passthrough files at /passthrough", "path", abs)
+		fs := http.FileServer(http.Dir(abs))
+		prefixHandler := http.StripPrefix("/passthrough/", fs)
+		mux.Handle("/passthrough/", prefixHandler)
+	}
+
 	mux.Handle("/v1/", handleGrpcGateway(c))
 
 	corsWrappedHandler := wrapHandlerWithCors(mux, props)